The ANSS event ID is ak0212umi8ne and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0212umi8ne/executive.
2021/03/03 05:35:26 61.453 -151.929 99.0 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/03/03 05:35:26:0 61.45 -151.93 99.0 4.5 Alaska
Stations used:
AK.CAPN AK.CUT AK.GHO AK.K20K AK.L19K AK.L20K AK.L22K
AK.M20K AK.N19K AK.O19K AK.PPLA AK.RC01 AK.SKN AK.SLK
AK.TRF AV.RED AV.SPCP TA.O22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 9.33e+22 dyne-cm
Mw = 4.58
Z = 110 km
Plane Strike Dip Rake
NP1 262 45 95
NP2 75 45 85
Principal Axes:
Axis Value Plunge Azimuth
T 9.33e+22 86 257
N 0.00e+00 4 79
P -9.33e+22 0 349
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.96e+22
Mxy 1.83e+22
Mxz -1.49e+21
Myy -3.35e+21
Myz -5.56e+21
Mzz 9.30e+22
- P ----------
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--############## ####################---
-############### T ###################----
################ ##################-----
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Global CMT Convention Moment Tensor:
R T P
9.30e+22 -1.49e+21 5.56e+21
-1.49e+21 -8.96e+22 -1.83e+22
5.56e+21 -1.83e+22 -3.35e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210303053526/index.html
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STK = 75
DIP = 45
RAKE = 85
MW = 4.58
HS = 110.0
The NDK file is 20210303053526.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 250 45 -90 3.78 0.2252
WVFGRD96 4.0 295 25 -40 3.84 0.2002
WVFGRD96 6.0 305 25 -25 3.86 0.2360
WVFGRD96 8.0 325 25 0 3.95 0.2626
WVFGRD96 10.0 325 30 0 3.97 0.2958
WVFGRD96 12.0 320 30 -5 4.00 0.3145
WVFGRD96 14.0 115 25 -30 4.02 0.3267
WVFGRD96 16.0 105 25 -45 4.05 0.3359
WVFGRD96 18.0 105 25 -45 4.07 0.3398
WVFGRD96 20.0 105 25 -45 4.09 0.3366
WVFGRD96 22.0 105 25 -45 4.12 0.3283
WVFGRD96 24.0 105 25 -45 4.14 0.3142
WVFGRD96 26.0 95 30 -55 4.14 0.2981
WVFGRD96 28.0 55 65 60 4.14 0.2928
WVFGRD96 30.0 55 65 60 4.15 0.2908
WVFGRD96 32.0 55 65 60 4.17 0.2871
WVFGRD96 34.0 50 65 55 4.18 0.2787
WVFGRD96 36.0 55 65 55 4.19 0.2700
WVFGRD96 38.0 225 75 45 4.23 0.2716
WVFGRD96 40.0 230 75 60 4.35 0.2847
WVFGRD96 42.0 230 70 60 4.35 0.2828
WVFGRD96 44.0 235 65 65 4.36 0.2853
WVFGRD96 46.0 245 60 80 4.37 0.2881
WVFGRD96 48.0 55 35 65 4.38 0.2950
WVFGRD96 50.0 245 60 75 4.40 0.3039
WVFGRD96 52.0 245 60 75 4.41 0.3137
WVFGRD96 54.0 240 60 70 4.42 0.3206
WVFGRD96 56.0 240 60 70 4.43 0.3250
WVFGRD96 58.0 65 45 70 4.43 0.3320
WVFGRD96 60.0 70 45 80 4.44 0.3545
WVFGRD96 62.0 70 50 80 4.46 0.3772
WVFGRD96 64.0 70 50 80 4.48 0.4023
WVFGRD96 66.0 70 50 80 4.49 0.4255
WVFGRD96 68.0 70 50 80 4.50 0.4473
WVFGRD96 70.0 70 50 80 4.51 0.4675
WVFGRD96 72.0 70 50 80 4.51 0.4857
WVFGRD96 74.0 70 50 80 4.52 0.5025
WVFGRD96 76.0 70 50 80 4.53 0.5175
WVFGRD96 78.0 70 50 80 4.53 0.5316
WVFGRD96 80.0 70 50 80 4.54 0.5439
WVFGRD96 82.0 70 50 80 4.54 0.5544
WVFGRD96 84.0 70 50 80 4.54 0.5637
WVFGRD96 86.0 70 50 80 4.55 0.5723
WVFGRD96 88.0 70 50 80 4.55 0.5791
WVFGRD96 90.0 70 50 80 4.55 0.5844
WVFGRD96 92.0 70 50 80 4.56 0.5894
WVFGRD96 94.0 70 50 80 4.56 0.5924
WVFGRD96 96.0 70 50 80 4.56 0.5956
WVFGRD96 98.0 70 50 80 4.56 0.5992
WVFGRD96 100.0 70 50 80 4.57 0.6013
WVFGRD96 102.0 70 50 80 4.57 0.6021
WVFGRD96 104.0 75 45 85 4.57 0.6030
WVFGRD96 106.0 70 50 80 4.57 0.6030
WVFGRD96 108.0 75 45 85 4.58 0.6039
WVFGRD96 110.0 75 45 85 4.58 0.6042
WVFGRD96 112.0 75 45 85 4.58 0.6033
WVFGRD96 114.0 75 45 85 4.58 0.6020
WVFGRD96 116.0 75 45 85 4.58 0.6013
WVFGRD96 118.0 75 45 85 4.59 0.6001
WVFGRD96 120.0 75 45 85 4.59 0.5969
WVFGRD96 122.0 75 45 85 4.59 0.5963
WVFGRD96 124.0 70 45 85 4.60 0.5937
WVFGRD96 126.0 70 45 80 4.60 0.5904
WVFGRD96 128.0 70 45 85 4.60 0.5888
WVFGRD96 130.0 70 45 85 4.60 0.5859
WVFGRD96 132.0 70 45 85 4.60 0.5826
WVFGRD96 134.0 70 45 85 4.61 0.5809
WVFGRD96 136.0 70 45 85 4.61 0.5758
WVFGRD96 138.0 70 45 85 4.61 0.5744
WVFGRD96 140.0 70 45 85 4.61 0.5700
WVFGRD96 142.0 70 45 85 4.61 0.5673
WVFGRD96 144.0 70 45 85 4.62 0.5639
WVFGRD96 146.0 70 45 85 4.62 0.5598
WVFGRD96 148.0 70 45 85 4.62 0.5554
The best solution is
WVFGRD96 110.0 75 45 85 4.58 0.6042
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3
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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00